In an integrated circuit, designers seek to increase the circuit density. In other words, designers seek to place more electronic devices in the same amount of space. The active devices are located in what is referred to active areas. The other areas are filled with insulators, spacers, or gaps that simply cannot be used due to the limitations of a particular layout design or the fabrication equipment.
In optical sensors, designers seek to increase the amount of space used for photodiodes (or any other type of optical sensor) as compared to other devices. This allows for larger photosites or for more photosites in the same amount of space, increasing the quality of the sensor output, or decreasing the total size of the sensor with the same quality, or both. For an optical sensor, increasing the amount of active area for the same amount of total area can allow for higher quality circuitry or for the space used for electronic devices other than photosites to be reduced.
For photodiodes and sensor arrays, as processes scale down and devices become smaller, the amount of charge accumulated by the photodiodes becomes smaller. As the level of signal is reduced, the signal-to-noise ratio becomes smaller. In order to maintain the same signal quality, the noise levels must also be reduced. One source of noise in sensor arrays is RTS (Random Telegraph Signal) noise, although there are other noise sources as well. RTS noise is caused, at least in part, by defects at interfaces between Si and SiO2 layers in the system. It is believed that charge carriers are trapped and detrapped at these interface defects. The measured charge at the other side of the defect will be increased or decreased randomly as charge flows across the defect. The noise can cause undesirable flickering pixels and increase the noise of the resulting images. While such noise can cause problems in a variety of devices, it has a noticeable effect with an in-pixel source-follower transistor. At low light levels, RTS from the source-follower is a significant noise source limiting imaging quality.
RTS noise at a source-follower, such as in in-pixel source-follower, arises at least in part from trapping and de-trapping of charge carriers under the gate oxide of the in-pixel source follower and of read out devices. For advanced semiconductor processing, gate oxide nitridation is done to impede the penetration of boron dopant atoms in polysilicon gate electrodes through underlying gate oxides. Boron penetrates into the poly gate electrode as part of the poly deposition process to form the gate electrode or as part of implantation processes after the poly gate is deposited but before it is patterned. Exposing the gate oxide to nitrogen reduces boron penetration through an oxide layer such as at a transistor gate. The nitrogen containing bond structure in the oxide may also improve the reliability of a gate oxide. However, the nitridized oxide layer also contains oxide-nitrogen-oxide bonds at and near the Si/oxide interface. The added nitrogen may also significantly increase the number of interface states and traps. This may result in higher RTS noise in an image sensor source-follower transistor as well as in other locations.
Gate oxides are typically nitridized in a processing furnace. A variety of noise reduction techniques are used to reduce the impact of nitridation on the resulting pixels. The most common solution to suppress the interface traps is to control the extent of the nitridation by controlling the temperature and gas mixture. Decoupled plasma nitridation (DPN) is also used which can place more of the nitrogen close to the poly/oxide interface at the top of the gate rather than at the oxide/Si interface at the bottom of the gate. However, there is still a nitrogen distribution tail that extends through even a thick gate oxide to the Si/oxide interface. DPN also has higher costs because it requires advanced processing tools.